Filters and particle separators for removing very small particulates from gaseous suspension are fairly well known in the art today. Although these devices work quite well under ideal laboratory test conditions, they all possess one major shortcoming when used in commercial, industrial, or military applications: when the airstream to be cleaned contains particles varying in size from 1 to 2 microns (the smallest particle visible to the naked eye is approximately 20 microns in diameter) up to several hundred microns, conventional interception or barrier type filters capable of removing the smaller particles will be quickly plugged by the larger particles contained in the air stream, quickly clogging the filter and necessitating its replacement or cleaning after only a very short period of operation.
An even worse case would occur when the airstream to be cleaned originated from an environment containing larger particles such as sand; such an environment would be mandated, for example, for a military vehicle capable of being operated in a desert environment. In such an environment, particle size can range up to a 2 millimeter diameter and down to as small as a 2 micron diameter, and it can be seen that under such circumstances the larger sand particle would quickly destroy a conventional filter, disabling the vehicle.
Devices for removing particles from an air stream are of five basic types: viscous impingement filters, electrostatic precipitators, dry interception or barrier filters, cyclone or vortex inertial separators, and a combination of an inertial separator and a barrier filter. A viscous impingement filter is made of a relatively loosely arranged filter medium, usually spun glass fibers, the surfaces of which are coated with an adhesive such as oil. Solid particles are thrown against the adhesive surface as the airstream passes through the filter, and are trapped in the filter. Viscous impingement filters are useless in removing smaller particles, since smaller particles tend to follow the path of the airstream through the filter, and thus are not removed from the airstream.
The second type of filter is the electrostatic precipitator, which passes the air stream through an ionized field which imposes an electrical charge on particles in the airstream. The ionized particles are then passed between a charged plate and a grounded plate, and are precipitated on to the grounded plate. Such electronic air cleaners possess many disadvantages, including their large size and awkward shape, severe limitations on the airflow which can be passed through a precipitator sized to be carried on a vehicle, the requirement of very high voltage power supplies, fairly high unit cost, and the necessity for frequent cleaning of the filter. Electronic precipitators thus are not practical for any use other than that in a stationary location.
The third type of device, the dry interception or barrier type filter, is by far the most common and the most economical particle separation device available. Barrier filters utilize a filter medium, such as spun glass fiber, which has a very fine filter diameter and a close weave to remove very small particles. While barrier filters are extremely efficient in the removal of very small particulates from an airstream, their efficiency is dependent on the absence of even a small number of somewhat larger particles. If such larger particles are present in the airstream, the barrier filter will become clogged very rapidly, necessitating replacement far too frequently to be practical.
Thus, it can be seen that in a barrier filter, two opposing considerations are present: first, it is desirable to make the filter highly efficient (close weave) in order to remove smaller particles in the airstream; on the other hand, the barrier filter must be made somewhat less efficient (wide weave) in order to prevent it from becoming clogged too quickly. These opposing considerations are irreconcilable in an airstream containing a variety of sizes of particles from which both large and small particles must be removed.
The fourth type of device widely used is the cyclone or vortex inertial separator, which operates on the principle that particles suspended in an airstream have greater inertia than the gas itself. The gas typically enters the separation chamber tangentially and leaves in an axial direction, although it is swirling at high speed in the chamber. Since the particles have a greater inertia than the gas, they are flung to the outer wall of the separator, from which they exit the device.
In a cyclone separator the gas is impelled into the separator with a high velocity. An alternate method uses a rotating member to induce the rotation of the airstream in the device.
Several mechanical devices operating on this principle have been manufactured, and are felt worthy of specific mention herein. U.S. Pat. No. 3,944,380, to Kampe, and assigned to the assignee of the present invention, is perhaps the best example of such a device. The Kampe patent discloses a nozzle for extracting dirt particles introduced into a radial inflow turbine. While the Kampe device is a highly efficient and useful device, it is virtually completely incapable of removing particles smaller than 10 microns, a limitation which is specifically cited in the specification. Other such devices are disclosed in U.S. Pat. No. 3,444,672, to Alsobrooks, and in U.S. Pat. No. 3,616,616, to Flatt, but these devices are also incapable of removing small particulates from the airstream, and are not designed to remove small particles from an airstream. The function of all three of the above-disclosed devices is to remove from the airstream particles which are large enough to present a serious erosion problem on high speed portions of jet turbine engines. Particles smaller than 10 microns need not be removed, because such particles do not present a serious erosion problem to high-speed metal surfaces, and thus do not present a premature wear threat to the devices.
Numerous other mechanical devices to separate particulates from an airstream are known in the art, but are of such a primitive state as to not merit specific mention herein.
Not only are cyclone or vortex inertial separators useless for removing small particles from an airstream, but they are generally of a fairly large size, weight, and cost, which when weighed with their low efficiency in removal of small particles, dictates against their use.
The fifth type of apparatus used for particulate removal is a combination of an inertial separator and a barrier filter. Unfortunately, such a combination, rather than presenting the advantages of both the inertial separator and the barrier filter, presents the disadvantages of both without the advantages of either. Since inertial separators are incapable of removing particles below 10 microns diameter from the airstream, these particles will pass through the inertial separator to the barrier filter. If the barrier filter is sized to remove particles as small as 2 microns diameter, it will become clogged by the particles of up to 10 micron diameter only slightly less quickly than if the airstream was not passed through the inertial separator at all. Therefore, the combination system will have not only the disadvantage of requiring frequent barrier filter replacement, but it will also have the disadvantage of the large size, weight, and cost of the inertial separator. Such a system would not be feasible in any but a stationary location.
Therefore, it can be seen that there are not existing filter or separator arrangements in the art which are capable of removing very small particulate matter suspended in a gaseous medium, which are also capable of removing larger particles from the gaseous medium and are available for use over extended periods of time without requiring shutdown to replace or clean filters. There exists today an urgent need for such a device, particularly in several vehicular applications.
The most obvious vehicular application of such a device is to remove particles from the airstream used to supply a motor vehicle operating in a dusty environment. While general commercial and industrial motor vehicles have generally adequate filter systems available, military vehicles which are designed to operate in an extremely dusty environment, such as in a desert field of operation, have as an essential requirement a particle separation system which can remove virtually all particulate matter from the engine air supply. It is obvious that such a military vehicle may not use a conventional barrier filter which will have to be replaced, since such replacement during field operations would be either impractical or impossible, since the vehicle would not be able to carry a sufficient supply of spare filters. In addition, it is extremely undesirable to have a military vehicle which would require frequent maintenance such as a changing of the engine air filter.
An additional, and highly important, military utilization for a particle separator capable of removing very small as well as larger particles from the air is in the cabin ventilation system of a sealed military vehicle. It is obvious that a military vehicle operating in a dusty or sandy environment will need a special ventilation system to supply the crew of the vehicle with an adequate supply of fresh air to breathe. Another reason mandating the separation of very small particles from the crew air supply is the possibility that such a military vehicle may be called upon to operate in a postnuclear environment. The primary carrier of radiation in such a post-nuclear environment are particles of dust, and conventional devices of the type described above don't get small particles out of the air supply. Such small particles, if they were included in the air supplied by a cabin ventilation system to the crew of the vehicle, would contaminate the cabin rapidly, causing radiation sickness and possible death to the crew members. Therefore, the particle separation device for such a cabin ventilation system must be capable of removing very small dust particles from the air circulated to the crew environment.
There are several possible commercial and industrial applications for a device capable of removing small as well as large particulate matter from a gaseous medium. The best example of such a requirement is in the exhaust system of a motor vehicle. Two particular environmental problems today are diesel soot from the exhaust of diesel engine vehicles, and lead particles from gasoline engines burning leaded gasoline. A conventional system would be useless in removing such particulates, since the device must not only be relatively small to be conveniently installed on the vehicle, but must be highly resistant to clogging over extended periods of time. There exists no particle separation device known today which is capable of lasting for an extended time in the exhaust system of a motor vehicle.
Therefore, it can be seen that it is highly desirable to construct a device which is capable of removing particulate matter ranging in size from sand particles to very tiny dust particles as small as approximately 2 microns from an airstream. It is desirable that such a device be utilizable as an engine intake air cleaner, removing particulate matter from the airstream supplied to a vehicle operating in a dirty, dusty, or sandy environment. Such a device must be capable of removing very fine dust particles from the environmental air intake supplying air to the crew of a vehicle; extremely small dust particles must be removed in such an application, since such dust particles are primary carriers of radiation contamination. Such a device may also be capable of removing lead or carbon particles from the exhaust stream from a vehicular engine, lasting for extended periods of time without maintenance.
A further requirement of such a device is that it be self-cleaning, that it need no replacement or cleaning of a filter element, and that it need only minimal maintenance and repair service over extended periods of use in hostile environments, including a desert environment and the exhaust stream of an internal combustion engine.
Since such a device may either be used in an application in which there may be an air pressure drop across it (such as in the intake system or in the exhaust system of an engine) or in an application where there is no inherent air pressure drop across the device (such as in an environmental ventilation system supplying air to a sealed cabin), it is desirable that the device operate in either a powered manner or that it be capable of being driven by the pressure drop across the device. The powered device should be driven by a small, efficient motor, such as a self-contained electric motor. The pressurized device may be driven by the pressure drop, and therefore must contain some type of driving means such as the turbine wheel in a turbocharger. The device must also be of minimal size, weight, and cost, since it may be utilized on vehicles as small as a commercially available passenger car. It must also be a bolt-on device, so it is necessary that the device be entirely self-contained.
Since the device is to be self-cleaning, and not requiring replacement or cleaning of a filter element, it must somehow scavenge the particles removed from the airflow, the particles being removed from the device to a collection area. In some applications, it must further scavenge to atmospheric pressure, even though the output of the device may be at less than atmospheric pressure. In the removal of the particles from the airflow, it is highly desirable that as little of the airflow as possible be used for purging the particles from the device, to make the device as efficient as possible.
A final desirable feature of such a device is that it inhibit the breakup of particles into smaller particles caused by the dynamic operation of the device. In order to do this, the particles contained in the airflow must be removed from the device as quickly as possible, and any secondary airflows within the device must be minimized or eliminated to prevent particles from bouncing around and breaking up within the device.